Composite phase change materials as passive thermal management systems have great potential application in power battery modules, but they are still limited by some drawbacks such as easy leakage, high rigidity, and low thermal conductivity. In this study, an excellent antileakage flexible composite phase change material has been prepared and utilized in the battery module. Styrene–butadiene–styrene and ethylene vinyl acetate with a synergistic effect are utilized to improve the antileakage and flexibility properties, which are beneficial to improve the thermal management effect of the battery module. Particularly, the mechanism of thermal contact resistance between the phase change material and battery module is deeply analyzed; the experimental results revealed that the maximum temperature and temperature difference can be maintained within 46.5 and 3.5 °C at 3C discharge rate, respectively. It indicates that the antileakage flexible CPCM can not only form a closer contact with the batteries but also exhibit excellent temperature controlling capacity, especially at a high discharge rate. This study can provide new insights into the decrease of the thermal contact resistance in the battery module; it will be suitable for other dynamic equipment such as energy storage power stations and firefighting power systems.